1. Field of the Invention
The invention relates to passive and active optical device materials. More particularly, the invention relates to a crosslinkable (meth)acrylate polymer composition which provides either passive or active wave-guide optical capabilities.
2. Description of the Related Art
Passive and active wave-guide optical device materials are key components for a wide range of cutting edge optical telecommunication devices. Also, signal processing by optical technology in broadband society will be a key issue to control large amounts of information accurately with fast response time. Particularly, there is a growing interest to use active nonlinear optical devices for signal modulation and switching. Also, passive optical wave-guide device materials are crucial components in order to lead optical signals into the active nonlinear optical devices. Organic active non-linear optics materials have several advantages, i.e. large NLO effect, nano- to pico-second response time, and structural design flexibility. Also, these polymer-based materials showed better processing ability, mechanical stableness, and cost effective compared to inorganic crystal material, such LiNbO3 and BaTiO3. Also, in term of response time and modulation speed, polymer-based materials have advantages than inorganic materials, because usually organic polymer-based materials have lower dielectric constant that leads to faster modulation and switching properties. Also, a passive material is a fundamental material for active optical devices, because this material can be used for the device portion in which optical signals can travel between devices and optical fibers.
It is desirable for polymer-based optical device material to have high stability (thermal, chemical, photochemical, and mechanical) and low optical loss along with high electro-optic performances.
For achieving high thermal stabilities, high Tg polymers matrix systems are desirable, such as polyimide, polyurethane, and polyamide. However, even in case of lower Tg polymer matrix system, such as poly(meth)acrylate, polystyrene, or polyolefine, crosslinking of the polymer matrix can improve and raise the Tg of the matrix polymer, which leads to higher thermal stabilities. Particularly, polyimides show excellent thermal stability and used for various engineering plastics materials. Since poly(meth)acrylate is very stable in chemical, mechanical and temperature properties and possesses excellent optical properties, its major interesting properties for passive or active optical devices include:
Chemical Stabilities
It is compatible with most microelectronics processes including photolithographic, Ion Reactive Etching (RIE), plasma and sputtering depositions, etc. It has reasonable solvent solubility therefore, it can be easily coated as thin film using variety of techniques (spin or spray coatings) before crosslinking.
Physical and Thermal Stabilities
Poly(meth)acrylate has a thermal expansion coefficient compatible with silicon, which will be very useful property for integration polymer optical devices with silicon based microelectronic devices. It is also chemically stable at temperature as high as 300° C.
Optical Properties
Poly(meth)acrylate has high optical transmission over a wide range from visible to telecommunication wavelengths. In optical wave-guide shape, the transmission loss is reported as lower as 0.1 dB/cm at 1.3 μm.
ElectroOptics Properties
When poly(meth)acrylate is loaded with chromophore, it becomes nonlinear poly(meth)acrylate material, and it could have relatively high nonlinearity.
Furthermore, particularly fluorinated polymers have unique features, such as low dielectric constants, low optical loss, and easier workability because of good solvent solubility. Usually, fluorinated Poly(meth)acrylate before crosslinking has very good solvent solubility so it is easily workable for spin-coating processing in fabrication of optical devices.
Also, dielectric constants are generally known the lower, as the more fluorine atom weight content ratio increased. Usually, the lower dielectric constant material can make optical signal traveling speed or modulation speed faster because of less π-electron interaction.
Generally, fluorinated polymer can reduce optical loss of signals. Optical propagation loss includes absorption and scattering losses. Material properties, namely interband electronic absorption of the chromophore and C—H vibration absorption of chromophore and polymer host, contribute to the absorption loss in the polymers. The scattering loss is mainly attributed to dust particles and micro domains introduced during the processing (spin coating, poling, photolithographic processing, and etc.). Therefore, advantages of the fluorinated polymer can mainly contribute to lower the absorption losses. Usually, the wavelengths which are generally used in the telecommunication are between 1.3 and 1.5 μm. Thus, if polymer-based materials contain significant amounts of C—H bonds, NH2, NH, or OH functional groups in the structure, these moieties may provide vibration absorption in the double frequency area that are significant and can give big influence on material absorption.
Crosslinked poly(meth)acrylate type material showed very good thermal stabilities and no critical deterioration. Sometimes, the second order nonlinear properties were observed more than 3000 hrs even at 100° C. at air. Thus, a combination of poly(meth)acrylate and fluorinated polymer resulted in satisfactory improvement as for optical device material. However, sometimes incorporation of chromophore into fluorinated poly(meth)acrylate resulted in lower thermal stabilities. So, in order to improve the thermally stabilities, a concept of crosslinking seems to be practical method to get higher and better thermally stableness after crosslinking.
In order to get good electrical optical performances, chromophores which are incorporated into matrix host materials are desired to orientate toward the same direction. The chromophore can be orientated to the same direction by poling process or some other proper processes. However, over the time, the direction of chromophore could be disorientated eventually. Particularly, these tendencies are seen in low Tg material case. In order to overcome this disadvantage, the concept of crosslinking is very helpful and practical method to get higher and better thermally stableness.
As typical crosslinking moieties, epoxy/isocyanate moieties and hydroxyl/amino groups are available. However, these kinds of moieties result in existence of NH- or —OH group, which contribute higher absorption in 1.3 to 1.5 μm wavelength region, after crosslinking. On the other hand, as examples of crosslinking moieties which do not result in undesired NH- or —OH group, tri-cyclization of acetylene group, cyanurate ring formation from cyanate ester derivatives, difluoro bismaleimide, or trifluorovinyl groups can be crosslinking moiety candidates. However, from standpoints of crosslinking temperature and easiness of synthesis, trifluorovinyl group seems to be most practical crosslinking moiety, because this group can crosslink around 160-200° C. enough lower than decomposition temperature of thermally unstable other components, such as chromophore.